1. Field of the Invention
The present invention relates to a radio transmitter using a Cartesian loop.
2. Description of the Related Art
In radio transmitters such as mobile terminals, a transmit RF signal is amplified by a power amplifier, to radiate the transmit RF signal from an antenna on the transmission side. The power amplifier is required to maintain linearity such that the transmit RF signal falls within a spectrum mask determined by the standard of the radio system. The power amplifier is also required to reduce the odd-order distortions. This is because the adjacent channel leakage power, which is one of the parameters specified by the standard of the radio system, is mainly caused by the odd-order distortions of the power amplifier.
Cartesian loops are used to reduce the odd-order distortions of the power amplifier and improve the linearity of the power amplifier. Cartesian loops are feedback loops, which includes a path which generates a baseband I/Q signals by branching part of the power of the transmit RF signal and inputting the power to a quadrature demodulator, and feeds the power back to the input of a quadrature modulator for transmission.
More specifically, part of the power of the transmit RF signal output from the power amplifier is transmitted to the feedback path of the Cartesian loop, and input to the quadrature demodulator through a variable attenuator and a low noise amplifier. Feedback I/Q signals obtained by the quadrature demodulator are combined (added or subtracted) with input I/Q signals. The combined I/Q signal is input to the quadrature modulator through a baseband amplifier. When the gain from the quadrature modulator to the quadrature demodulator is sufficiently larger than 1, the gain from the input end of the quadrature modulator to the output end of the power amplifier is determined by the attenuation amount of the feedback path of the Cartesian loop, and the linearity of the transmit RF signal depends on the linearity of the feedback path. Therefore, it is necessary to enhance the linearity of the feedback path.
On the other hand, since mobile terminals operate with a built-in battery used as a power source, IC low-power-consumption technology is indispensable to reduce exhaustion of the battery and extend the communication time. In mobile terminals, the power consumption of the power amplifier for transmission is the largest. With increase in packing density of the radio unit used for mobile terminals, recently the radio units have been manufactured by complementary metal-oxide semiconductor (CMOS) technology, which is suitable for cost reduction. Generally, although power amplifiers having a CMOS structure can achieve cost reduction, the efficiency thereof decreases. Cartesian loop technology is linearity increasing technology for the radio unit as described above, but also can be regarded as technology for increasing the efficiency of the power amplifier. Therefore, the efficiency of the power amplifier or the radio unit can be increased by applying a Cartesian loop to the radio unit of the CMOS structure.
Since Cartesian loops have a feedback path, it is important how the operation stability is secured. To secure the stability of Cartesian loops, JP-A H10-136048 (KOKAI) proposes a method of controlling the phase of a local signal supplied to the quadrature demodulator in the feedback path, in accordance with the phase difference between the feedback I/Q signals and the input I/Q signals. However, in JP-A H10-136048 (KOKAI), it is not considered to render the power (transmission power) of the transmit RF signal variable.
When a Cartesian loop is used, it is possible to reduce the power consumption of the whole transmission apparatus even in consideration of the power consumption of the feedback path in high output of the transmit RF signal. On the other hand, in low output of the transmit RF signal, the linearity of the power amplifier and the driver amplifier driving the power amplifier is high, and thus it is not necessary to increase the linearity thereof by using a Cartesian loop. When a Cartesian loop operates also in low output, the efficiency decreases due to the power consumption of the feedback path. Therefore, to efficiently perform transmission power control used in recent radio systems, it is desirable to close and open the Cartesian loop according to the transmission power, from the viewpoint of reduction in power consumption.
When a Cartesian loop is opened and closed according to the transmission power, a transient response time since the loop is closed until the transmission power reaches a predetermined value increases. For example, when an average output of the input I/Q signals before the loop is closed is set to 100 mV, supposing that the gain of the variable attenuator in the feedback path is set such that the average output of the feedback I/Q signals is 1000 mV, the transmission power when the loop is closed is different from the transmission power when the loop is opened. Therefore, the time necessary for convergence of the transmission power since the loop is closed is increased. When operation is performed to set the transmission power by controlling the gain of the variable attenuator in the feedback path after the loop is closed, the convergence time of the transmission power is further increased. JP-A H10-136048 (KOKAI) does not disclose a concept of transmission power control, and thus does not disclose any method of reducing the convergence time of the transmission power in closing and opening of the Cartesian loop.